President Donald Trump drew a red line on Jan. 2, warning that if Iran’s rulers answered mass protests with mass murder, the United States would respond militarily. Since then, Ayatollah Ali Khamenei has called the bluff. The regime’s security forces have cracked down, killing thousands and detaining more.

That leaves Trump with a decision that matters beyond Tehran. If the U.S. allows a dictator to murder civilians after a direct presidential warning, as President Barack Obama did after drawing a red line in Syria, America will lose its ability to deter. American presidential warnings will be seen as empty noise. In the Middle East, weakness is not interpreted as caution but as permission.

A full war to change the Iranian regime would be folly and is doubtless not being contemplated. But Trump must take measured military action and soon.

Sanctions alone won’t cut it. Iran is already heavily sanctioned. The Islamic Republic has........

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